This invention relates to methods and systems used to characterize subterranean formations.
In order to make effective decisions about drilling and producing hydrocarbons from a subterranean formation, geologists and reservoir engineers attempt to create a static and a dynamic three-dimensional description of an oil or gas reservoir, based on the one- and two-dimensional data from well cores and seismic surveys. The objectives of a static reservoir description include 1) extrapolate core data to uncored wells; 2) define quantity and distribution of rock properties such as porosity, saturation, and permeability in each well; 3) interpolate rock property data between wells; 4) identify flow units from porosity versus permeability populations and 5) build a knowledge base about the reservoir for today and the future. The objectives of a dynamic reservoir description are 1) test the static model for accuracy; 2) predict future performance under various operational scenarios and 3) optimize production for maximum long-term economic return.
Analysis of cores is one method used to obtain static reservoir data. In this method a number of boreholes are drilled over the area of land under which the subterranean formation of interest is located and well cores are extracted. These well cores are then tested and various physical, chemical, electrical, or other properties of the rock and/or fluids are recorded from analysis of the cores. Typically, a well log is stored in a data set in a computer database that can be displayed on a computer display, paper graph or other display medium with the measured physical property of the rock on one axis and depth (distance from the surface) on the other axis. More than one property is typically displayed on the same log.
Analysis of a single core provides information about facies through which the borehole was drilled as a function of depth at the specific surface location of the bore hole. By plotting similar properties over a wide area, the geologist or reservoir engineer can build a three dimensional model of where particular facies exist, make estimates of where hydrocarbons are likely to be present and estimate the quantity of hydrocarbons that can likely be recovered.
Cores may be analyzed in a number of ways. Physical tests may be performed on the core to measure its bulk density for example. Chemical analysis can be performed to estimate effective atomic number. Other laboratory tests can also be performed.
Digital analysis of cores is becoming more common. Dual energy X-ray computed tomographic image scans of cores are used to estimate bulk density and effective atomic number. Plugs extracted from cores are used for further analysis to estimate rock properties such as resistivity, elasticity and permeability and rock/fluid properties such as relative permeability and capillary using digital rock physics techniques.
Using cores to estimate properties of subterranean formations has several shortcomings. Some underground formations such as shales can have many very thin facies, sometimes only a few centimeters thick. The accuracy of core depth estimates is on the order of 3 meters. Boreholes can be horizontally separated by hundreds or thousands meters on the surface. Each borehole provides a point of information about the underground formation at a specific surface location. The geologist must interpolate between borehole locations to estimate the location of a facies of interest in between borehole locations. Underground facies typically do not follow straight lines and as such, significant errors in estimating location of facies can occur. With the advent of horizontal drilling the need to have more detailed information about the precise location of facies and facie properties has become more important. It is not practical to extract horizontal cores from a well bore and vertical cores provide only limited data. Core analysis is not practical in real time or near-real time. Cores must be extracted and shipped to a laboratory for analysis and this can require many days or weeks to complete. As a result, core analysis is of very little value to questions that arise at the time a well is being drilled.
Other techniques are employed to gain insight into wells and subterranean formations. Wire line logs employ a measuring instrument, often called a probe, sonde or logging tool, that is lowered into the borehole on the end of an insulated electrical cable. The cable provides power to operate the downhole instrumentation and additional wires in the cable carry signals from the tool back to the surface. The cable itself is used for estimating depth such that properties measured by the tools can be related to depth in the borehole. Wire line logs are limited because the measurements must be performed down hole and the type and quality of measurements that can be made are limited relative to laboratory tests. To use a wire line log, the drill must be removed from the borehole. Wire line logs have the advantage that they can provide current information about down hole conditions of a well that is being drilled but the need to remove the drill to perform the wire line test delays drilling, often at significant cost.
Mud logs are another technique used to examine well characteristics. Mud logs gather qualitative and semi-quantitative data from hydrocarbon analyzers that measure and record the level of hydrocarbons brought up in the mud. Analyzers such as chromatographs are used to determine the chemical makeup of the hydrocarbons. From the mud analysis, information about the formation can be estimated. In addition to drilling mud, drill cuttings are carried to the surface in the drilling mud. These cuttings can be examined and tested for composition, size, shape, color, texture and hydrocarbon content. Because the mud flows from downhole to the surface and because the drill cuttings must be fluidized in the mud to be returned to the surface, there is a lag time and error in knowing the exact location of the mud and cuttings. Lag times can vary from several minutes to several hours depending upon well conditions. Leaks in the drill pipe or fractures in the well bore can affect lag time introducing additional error in estimating depth from which mud or drill cuttings came. In addition to the error in exact location, drill cuttings are small, typically from about several millimeters to less than 250 microns in size. The small size and uncertainty about the location from which the cutting was produced limit the value of information from analysis of cuttings.
Logging while drilling (LWD) is another technique in which well logging tools are placed downhole as part of the bottom hole assembly (BHA) while the well is being drilled. LWD measurements are somewhat more limited than wire line logs but the cost of LWD is relatively high.